Recall that we have the boolean operators and (&&) and or (||) in Ruby. The && operator only returns true when the expressions on both sides of the operator are true; || returns true when one or the other or both of the expressions involved are true.

Ruby does this via short-circuit evaluation. That means that Ruby doesn’t look at both expressions unless it has to; if it sees

false && true

it stops reading as soon as it sees && because it knows false && anything must be false.

Remember how Ruby returns the result of the last expression it evaluated? We can use that to show short-circuit evaluation in action.


Check out the code in the editor, then click Run. Because only false and nil are false values in Ruby, both strings are treated as true. Ruby knows true || anything is true, so in a || b, it only evaluates a. Since it might encounter a false in the b part of a && b, however, it has to evaluate b, which we see in the result!

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